The present invention relates to a novel metal-organic polymer composite structure with high potential application in electronic materials, magnetic materials and optical materials and other purposes, and a process for producing the same.
The incorporation of ultrafine metal particles of nanometer size (metal nanoclusters) into a polymer (an organic polymer) to make a composite is an important technology for creating functional materials having new functions imparted thereto, such as electroconductivity, optical properties (linear or nonlinear), magnetic properties and the like.
For making such metal-organic polymer composite structures, there have hitherto been studied processes in which metal ions are selectively disposed in one of the phases constituting a microphase-separated structure from a block copolymer, followed by reducing the metal ions to the atomic metal by any suitable measure. The following is an overview of such conventional processes:
(1) A block Copolymer Composed of Polystyrene and Polyethylene Oxide (PS-b-PEO)
This is a process in which a block copolymer composed of polystyrene and polyethylene oxide (PS-b-PEO) is dissolved in a dilute toluene solution to form a micelle, a metal complex is formed in the core of the micelle by reacting chloroaurate ion with the PEO, and then gold fine particles with a diameter of about 4 nm are formed within the PEO phase through the irradiation of the metal ion complex with an electron beam [J. P. Spatz, A. Roescher, and M. Moeller, Adv. Mater. 8, 337 (1996); A. Roescher and M. Moeller, Polym. Mater. Sci. Eng. 73, 156 (1995); A. Roescher and M. Moeller, Polym. Mater. Sci. Eng. 72, 283 (1995)]. However, this process has a drawback in that the fine gold particles in the resultant structure are limited to ones with a hexagonal arrangement, due to the stable spherical micelle of the PS-b-PEO formed in the toluene solution.
(2) A Block Copolymer Composed of Polystyrene and poly(2-vinylpyridine) (PS-b-P2VP)
Chloroauric acid is added to a dilute toluene solution in which there has been dissolved a block copolymer composed of polystyrene and poly(2-vinylpyridine) (PS-b-P2VP), to prepare a solution containing chloroauric ion. To the resultant mixed solution is added hydrazine (N2H4) as a reducing agent to form fine gold particles [J. P. Spatz, S. Mossmer, and M. Moeller, Chem. Eur. J. 2, 1552 (1996)]. In this method there is a need to remove hydrazinium chloride (N2H5Cl), produced as a by-product in the solution, from the solution containing the fine gold particles.
(3) A Process Via the Complex Formation of Metal Ions with a Block Copolymer Containing a Phosphine Moiety
A block copolymer containing a phosphine moiety is prepared, and then a metal salt is selectively disposed in the phosphine moiety, followed by a solvent casting to form a cast film. Heating the film to about 90xc2x0 C. produces a composite film containing ultrafine metal particles such as silver particles [Y. Ng Cheong Chan, R. R. Schrock, and R. E. Cohen, J. Am. Chem. Soc. 114, 7295 (1992)], gold particles [Y. Ng Cheong Chan; R. R. Scrock, and R. E. Cohen, Chem. Mater. 4, 24 (1992)], and palladium or platinum particles [Y. Ng Cheong Chan, G. S. W. Craig, R. R. Schrock, and R. E. Cohen, Chem. Mater. 4, 885 (1992)]. This method may require much time for preparing the block copolymer containing a phosphine moiety.
(4) A Process in which Selective Incorporation of Metal Ions is Effected Through the Cross Linking of a Microphase-separated Structure Formed from a Block Copolymer
The P2VP [poly(2-vinylpyridine)] domains of microphase-separated structure from a block copolymer composed of polystyrene and poly(1-vinylpyridine) (PS-b-P2VP) are crosslinked using 1,4-diiodobutane for the chemical fixation of the P2VP domains as well as the incorporation of iodine ions into the P2VP domains. The reaction of the iodine ions with silver ions results in the incorporation of a silver salt, followed by photo-irradiation to reduce the silver salt to silver nanoclusters present inside the P2VP domains [B. Saito, H. Kotsubo, and K. Ishizu, Polymer 33, 1073 (1992); R. Saito, S. Okamura, and K. ishizu, Polymer 33, 1099 (1992); R. Saito, S. Okamura, and K. Ishizu, Polymer 34, 1183 (1993); R. Saito, S. Okamura, and K. Ishizu, Polymer 34, 1189 (1993)]. The crosslinked PS-b-P2VP may be dissolved in a mixed solvent of 1,4-dioxane and an aqueous solution of silver nitrate. The resultant solution is then subjected to a step of forming a film so that silver nitrate is selectively contained in the P2VP domains, followed by a step of reducing the silver nitrate through photo-irradiation to form silver nanoclusters [R. Saito and K. Ishizu, Polymer 36, 4119 (1995)].
Another example of the processes for incorporating ultrafine metal particles into one of the phases of a block copolymer includes the one as described in [R. W. Zehner, W. A. Lopes, T. L. Morkved, H. Jaeger, and L. R. Sita, Langmuir. 14, 241 (1998)].
All of the above-mentioned processes need highly complicated steps for producing the composite structure (particularly the step for the reduction). In addition no particular attention has been paid to arranging such ultrafine metal particles in an orderly manner in the resultant composite structure and no measures have been proposed for this purpose.
It is an object of the present invention to establish a technology for obtaining a new type of metal-organic polymer composite structure, by taking advantage of a microphase-separated structure formed from a block copolymer, which is quite easy to prepare and in which ultrafine metal particles are arranged in an orderly manner in one of the polymer phases of the microphase-separated structure.
According to the present invention, the above-mentioned object is achieved by providing a metal-organic polymer composite structure comprising a microphase-separated structure from a block copolymer composed of two or more polymer chains which are incompatible with each other bonded in an end-to-end manner, wherein one of the polymer phases of said microphase-separated structure selectively contains ultrafine metal particles arranged in a row or rows. In a particularly preferred embodiment of the present invention, the microphase-separated structure from the block copolymer is of a lamellar structure.
According to the present invention there is also provided a process for producing such metal-organic polymer composite material, which process comprises (i) the step of dissolving xe2x80x9cmetal ionsxe2x80x9d and xe2x80x9ca block copolymer comprising one or more polymer chains having an affinity for the metal and, bonded thereto in an end-to-end manner, one or more polymer chains having no or little affinity for the metalxe2x80x9d in a mixed solvent of xe2x80x9ca high-boiling solvent having the ability to reduce the metal ion and having a boiling point of 80 to 200xc2x0 C.xe2x80x9d and xe2x80x9ca low-boiling solvent having a boiling point of 10 to 80xc2x0 C.xe2x80x9d; (ii) the step of removing the low-boiling solvent at the boiling point thereof to cause the block copolymer to form a microphase-separated structure; and then (iii) the step of reducing the metal ions while removing the high-boiling solvent at the boiling point thereof. A case is here excluded where both the boiling point of the low-boiling solvent and that of the high-boiling point are 80xc2x0 C.